CH. XII.] SPECIFIC GRAVITY. 271 



B. The Physical Chemistry of the Urine. 



/. General Properties. 



Normal human urine is a clear yellowish fluid, the 

 depth of the tint depending largely on the concentration. 

 On standing, a cloud (nubecula) of mucoid containing 

 epithelial cells separates out. After a heavy meal urine 

 may be passed cloudy, due to earthy phosphates and 

 carbonates. On standing, these settle to the bottom of 

 the vessel as a white deposit, insoluble on warming, but 

 soluble in acids. 



Also on standing a cloud of urates may settle as a 

 reddish deposit that clears up on warming. 



Fresh urine has a characteristic odour of the aromatic 

 type, due to the presence of some substance that has 

 not yet been recognised. On standing, an unpleasant 

 ammoniacal odour develops as the result of bacterial 

 decomposition. 



//. The Specific Gravity. 



Usually lies between 1012 and 1024 (water = 1000). 

 With copious drinking it may fall to 1002. After excessive 

 perspiration it may rise to 1040. 



The determination of the specific gravity for clinical 

 purposes is most conveniently made by means of a urino- 

 meter, a weighted cylinder that floats in the urine. The 

 depth to which it sinks depends on the density of the 

 fluid, and this can be read directly by means of a graduated 

 scale on the stem. The instrument is calibrated for a 

 certain temperature, usually 15 C. 



The urine should be either cooled or warmed to this 

 temperature, or a correction made by adding i unit for 

 every 3 degrees above this, or subtracting i for every 3 

 degrees below the standard. Thus, if the reading be 1018 

 at 1 8 C., the corrected Sp. Gr. is 1019. 



